The workpiece and the tool after the process is complete IPH - Hannover
The workpiece and the tool after the process is complete.

Pioneering a New Forming Concept

Longitudinal axis symmetrical pre-forms Three types of defects EC funds research Process monitoring

Cross-wedge rolling is a well-established process used by forgers to produce cylindrical pre-forms, with very efficient material utilization. To prevent the appearance of material defects during forming, the wedge geometry of the tools is adjusted to the designated pre-form.

After cross-wedge rolling, a workpiece is shown with the wedge tool.

Today, cross-wedge rolling is executed within the hot temperature range. To analyze warm cross-wedge rolling, the scientists at IPH — Institut für Integrierte Produktion Gemeinntzige GmbH developed a test site for cross-wedge rolling experiments.

Cross-wedge rolling
Cross-wedge rolling is used to convert cylindrical billets into longitudinal axis symmetric pre-forms. The billet is formed by two wedges, moving tangentially to the work piece. The method is well suited for forming conical axles. A cross-wedge roll consists of two or three tools that are clamped onto the rollers. The work piece is inserted into the roll gap in the longitudinal direction. Figure 1 shows a conventional cross-wedge rolling machine with round tools.

The test site used in the IPH development uses flat tools, because these are easier to design and manufacture. A rolling wedge may consist of up to four zones: In the first zone – the knifing zone – a slit-shaped cut is centered upon the billet. The second zone is used as a guide zone, where the billet rotates around its axis to generate an even cut. Within the subsequent stretching zone, the central cut is dilated to a designated value and the billet is stretched in longitudinal direction. Within the terminal sizing zone the billet is, at least once, rotated around its longitudinal axis [Doe07]. Thus, an evenly formed work piece is produced. Compared to more conventional production methods, for example, turning, die forging or casting, cross-wedge rolling offers many advantages due to its
• High productivity,
• High material efficiency,
• Low operational costs,
• Easy machine concept,
• Low energy consumption, and
• No lubrication and coolants [Her97].

Figure 1: Conventional cross wedge rolling machine with round rollers and tools. [Las10]

Despite these advantages, cross-wedge rolling is not widely used for industrial purposes because the design and construction of the base tools for cross-wedge rolling requires expert knowledge. The challenge is to prevent possible failure mechanisms during rolling.

Johnson and Mamalis discern three types of part defects: surface defects, internal defects and imperfect shape [Joh77]. The formation of helical dents or necks and folds are among the surface defects. Internal cavities and part-fractures are considered to be internal defects. Imperfect shapes are caused mostly by a defective wedge design, which results in a deviation of the finished part geometry from the required net shape. These defects can lead to sudden part-failure during operation.

Failure mechanism generation and its impact on parts during cross-wedge rolling were investigated using practical tests and the finite-element method for calculations. Based on numeric and experimental testing-methods, the development of defects can be construed as a function of raw material properties and the three primary parameters of cross-wedge rolling: forming angle, extension angle and reduction of diameter.

Another influential factor is slippage between the part and tool during rolling [Gla98, Pat98]. If the slippage is too great, the work piece is extruded between the wedges without rotating, and thus its axial formation is incomplete. The friction between work piece and tool is directly tied to the slippage. The higher the friction, the smaller this slippage will be.

Cross-Wedge Rolling within the Warm Temperature Range

Currently, cross-wedge rolling is used only within the hot temperature range. Until now, the application of this process using warm temperatures was not the subject of scientific evaluation. The geometric spectrum of warm forged parts is limited by the mass distribution. The higher the degree of variation along the centerline, the more forging operations are required. Due to the cooling of the parts during the respective forming steps, these additional forging operations have a negative effect on achievable tolerances [Beh08].

Figure 2: Flat-die cross wedge tool with three forming zones.

To overcome these geometrical restrictions, the European Commission funded a research project — Development of a Variable Warm Forging Process Chain (DeVaPro), [Beh09] — which aims to develop an adaptive pre-forming process. Once the pre-forming of semi-finished products via cross-wedge rolling in the warm temperature range in combination with a defined reheating has been implemented, a complete process chain for warm forging of long parts can be applied.

To date, pre-forming via warm rolling has not been realized, neither by manufacturers nor by research scientists. One reason for this is the lower temperatures, and thus the necessity of higher forming forces compared to hot forming. In view of an expansion of the geometric spectrum of warm forged parts, the transfer of the warm forming concept to cross-wedge rolling is the key challenge. Industrial rolling processes within the hot temperature range are located around 1,250°C.

Figure 3: Lower tool system for cross wedge rolling experimental rig.

A secondary goal of the research project is to design a cross-wedge rolling process within the warm temperature range, at approximately 900°C. Compared to hot-formed work pieces, warm-formed parts offer superior roughness values of 20 ∝m at most, less surface decarburization, and improved dimensional accuracy of IT 10 to IT 11[Beh08].

Test site for Parameter Studies

For experimental studies during the design of a cross-wedge rolling process within the warm temperature range, a test site was developed at IPH. One goal is to study the impact that different temperatures and wedge geometries have upon the raw materials and billet diameters in cross-wedge rolling. The test site is integrated into a hydraulic press. The wedge segments are designed as flat wedge tools and are moved in a linear direction by the power of hydraulic cylinders. This design is characterized by its flat, and thus structural and production-oriented, easy build-up. Compared to a curved roll the design is less complex, despite having the same design parameters.

Test Rig
The test rig is shown in its starting position, with the workpiece in place on the cross-wedge tool, with the tool fixture below.

The lower part of the tool-system for the cross-wedge rolling test site is depicted Figure 3 and can be explained as follows: Different wedge segments can be fitted into the tool fixture and thus, various wedge geometries can be evaluated. To reduce heat transfer between part and tool during warm forming, the tool fixture was equipped with heating cartridges which, by heating the tool fixture, heat the wedge segment as well. The tool fixtures are driven by two hydraulic cylinders and are outfitted with a pivoting pillow block for mounting the cylinder.

Thus, the force lines of drive and agent are in-plane during the rolling process. Thereby, transverse forces and moments occurring around the vertical axis are avoided. A joint head, to offset displacement and overturning, is connected with the pivoting pillow block by a bolt. To record the forces necessary to move the tool, force sensors are installed between the piston rods and the hydraulic cylinders. Each sensor transmits the measured data to a measuring computer, which displays the force progression of the hydraulic cylinders over the path of the tool slide as a diagram. Thus, the experimental rolling process can be monitored.

The cross-wedge rolling tool is controlled with flat guides. These are characterized by high power intake and load capacity, owing to large contact and support areas. The high friction and the resulting wear of the flat guides can be minimized by separating the contact areas via lubrication. Tangential forces are absorbed by lateral bearing surfaces. Radial forces are absorbed by the hydraulic press during the forming process. Thus, all forces occurring during cross-wedge rolling are reliably guided and absorbed.

In addition to the lower part, the tool system for the cross-wedge rolling test site is comprised of an upper part similar to its counterpart and fastened to the upper ram of the hydraulic press. Using the upper movable plunger, the height of the roll gap can be adjusted during forming. Thus, different part diameters can be evaluated.

To limit the investment in process equipment for the cross-wedge rolling test site, the part and wedge geometries derived from the industrially rolled parts in the DeVaPro project were scaled down. The challenge was to downsize the cross-wedge rolling process and yet to conserve its comparability to the original size. Due to its impact on the temperature distribution within the part, the changes in the parts’ surface-to-volume ratio proved to be a critical factor for geometrical scaling. The temperature distribution within the downsized part ought to be qualitatively similar to the values within the original sized part. By using FEM simulation with the FORGE3 program, a scaling factor of 60% was identified as a workable compromise between size and similarity. This scaling factor permits a comparable depiction and thus experimental studies of the rolling process in combination with marginal technological and financial expenses.

The test site designed for cross-wedge rolling allows practical evaluation of the impact different work piece and tool temperatures, as well as tool geometries and rolling speeds, have on the forming result. In the future, the deliverables of the practical rolling tests will provide the expertise to design customized wedge geometries, forming speeds and tool-temperatures for hot and warm cross-wedge rolling. Within the scope of the DeVaPro project, the experimental rig contributes significantly to the implementation of a complete process chain within the warm temperature range.

Furthermore, the concept to integrate such a cross-wedge rolling machine with flat wedge tools in an existing hydraulic press allows forging operators to roll small pieces in small, cost-efficient batches.

The authors thank the European Commission for the funding of this project in the 7th Framework Programme. All of them are researchers affiliated with the Institute of Integrated Production Hannover, in Germany. Visit www.iph-hannover.de

References
[Beh08] Behrens, B.-A.; Suchmann, P.; Schott, A.: Warm forging: new forming sequence for the manufacturing of long flat pieces. In: Production Engineering, Springer Verlag Berlin, 2. Jg. (2008), H. 2, Nr. 3, S. 261-268.

[Beh09] Behrens, B.-A. et al.: Development of a Variable Warm Forging Process Chain. www.devapro.de, 19.10.2009.

[Doe07] Doege, E.; Behrens, B.-A.: Handbuch Umformtechnik – Grundlagen, Technologien, Maschinen. Springer-Verlag, Berlin, Heidelberg 2007.

[Gla98] Glab, R. et al: Process of Partial Bulk Metal Forming - Aspects of Technology and FEM Simulation. In: Journal of Materials Processing Technology, vol. 80-81 (1998), pp. 174-178.

[Her97] Herbertz, R.; Hermanns, H.: Querkeilwalzen. Ein wirtschaftliches und flexibles Vorformverfahren fr die Groserienfertigung. In: Schmiede-Journal, Industrieverband Massivumformung e. V., o. Jg. (1997), H. 2, S. 20-21.

[Joh77] Johnson, W.; Mamalis, A.G.: A survey of some physical defects arising in metal working processes. In: 17th International MTDR Conference, IFC, Ltd., eds., 1977, London, pp. 607-621.

[Las10] LASCO Umformtechnik GmbH; www.lasco.de; 2010

[Pat98] Pater, Z.: Simulation of Cross Wedge Rolling Process using the Upper bound Method. In: Scandinavian Journal of Metallurgy, vol. 27 (1998), no. 3, pp. 102-127.

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